1. Field of the Invention
This invention relates to turbine wheel flow measuring transducers and, more specifically, to flow meters that measure low rates of flow of corrosive fluids, both liquids and gases.
2. Description of the Related Art
Conventional flow tube type liquid and gas flowmeters took the form of a vertically mounted glass tube containing a spherical or other shaped float that moved upward in the glass tube in proportion to the flow rate of the liquid or gas flowing through the glass tube. Such flow tube type flowmeters have historically been employed in various liquid and gas analyzers, liquid and gas metering devices and laboratory apparatus requiring flow measurement.
A great majority of instrument-related liquid and gas flow measurement applications call for very low flow rates to be measured. The majority of all such flow rate requirements was within the 100 to 1000 milliliter range with almost all falling within the 20 to 10,000 milliliter/minute flow range. Accordingly, a flow meter having the following characteristics would be highly desirable: inexpensive to construct; simple and reliable design; capable of measuring flow rates from 20 ml/minute to 10,000 ml/minute; having a very low pressure drop across the flow transducer; linear 0 to 5 v. D.C. electrical output directly proportional to the flow rates; and small in size, low in power consumption, with no "warm-up" time.
The axial flow turbine type flow transducer (Norton in HANDBOOK OF TRANSDUCERS FOR ELECTRONIC MEASURING SYSTEMS, first published in 1969) meets at least some of the requirements and was originally developed for aerospace flow measurements but has since become popular in numerous other fields. The typical turbine rotor resembles a propeller blade suspended inside a tube so that as a gas flow moves through the tube, the turbine rotor spins in proportion to flow rate. Bearing friction becomes a paramount problem whenever gas flows below 1000 ml/minute are to be measured. So, as sensitivity for this type of turbine gas flow meter increases, then the costs related to construction to overcome frictional problems accordingly also increased since rotor blade balancing problems were accomplished by tedious hand methods.
It is desirable to have a turbine-type flow measuring transducer that produces precisely linear pulses and direct current voltage outputs (0 to 5 volts D.C.) in response to flow rates. Further, the design of the flow sensor should be compact so that it may be employed inside various types of analytical instruments. The design should also be readily adaptable so that the flow sensor may be converted from measuring one flow range to measuring another, different flow range. Desirably, the design should allow the 5:1 linear range on gases and at least a 10:1 linear range on very low flow rate liquids, and up to about 50:1 range on higher flow rate liquids. Further, the power requirement should be for a single D.C. power supply at less than 200 miliwatts.
U.S. Pat. No. 4,467,660 provides a turbine wheel flow measuring transducer that meets many of these requirements. The apparatus of the '660 patent measures low flow rates of gas. A very thin, small diameter disk is rotatably mounted in a chamber within a housing through which the gas to be measured passes. Plural small reaction turbine blades or teeth are formed around the periphery of the disk for receiving substantially constant impact of the gas entering the chamber. A nozzle inlet means mounted in the housing directs the gas entering the chamber against the teeth on the disk, causing the rotation of the disk. A photoelectric circuit directs light onto side portions of the disk to measure the relative movement of the disk in response to the impact of the gas against the reaction turbine blades on the disk. The disk has reflective surfaces formed on the side portions for reflecting the light directed from the photoelectric circuit so that light reflected may be photoelectrically detected and an electrical measure of the gas flow rate formed.
The device of the '660 patent provides a reaction turbine wheel which is sufficiently sensitive to rotate with gas flow rates for air at low flow rates, such as, as low as 20 ml/minute, and possibly lower. Sensitivity for liquids is as low as 10 ml/min, and possibly lower. The impact torque imposed upon the turbine wheel by the gas or liquid must exceed the frictional counteractive torque caused by the weight of the turbine wheel assembly resting upon the shaft bearing supports, so that flow rates at this low level can be measured.
The apparatus of the '660 patent, is, however, not corrosion resistant. And, it is desirable to develop a corrosion resistant flow sensor capable of measuring very low flow rates of low viscosity liquids and gases that are very corrosive or that require the highest level of purity. Thus, the materials of construction of the device should prevent contamination of fluids being measured. Also, this device consumes approximately 320 miliwatts.
While there are numerous gas and liquid flow sensors capable of meeting some of the objectives enumerated above, none, to the inventors' knowledge, will meet all of the objectives outlined. For example, Brooks Instruments and Molytech both manufacture liquid flow sensors based upon a thermal detection principle. Most of these consume several watts of power. These sensors can measure very low liquid flow rates and are three to five times as expensive as turbine designs. Currently they are only available in designs with extremely low flow rate ranges (generally below 100 ml/min). Some other liquid thermal sensors use stainless steel which cannot handle many corrosive liquids. One European manufacturer makes a small turbine wheel sensor out of KYNAR.RTM. plastic. While this flow sensor has a lower flow rate measuring limit of about 100 ml/min. in liquids, it is unsuited for measuring low flow rate gas flows. Additionally, while KYNAR will withstand chemical attack from a variety of aggressive chemicals, it is subject to attack by many other chemicals. Also, this turbine wheel design of the sensor does not allow a turn down to measure very low liquid flow rates (of the order of 10 ml/min. or less). Finally, the design is not readily adaptable to having a large variety of flow ranges. Components must be remolded in order to measure different flow ranges. Another company, Miniflow Systems, Inc., makes a liquid flow sensor that has no shaft upon which a turbine wheel spins. This is called a "bearing less turbine wheel flow-sensor". The sensor works only in liquids and produces only a pulse output signal. The flow sensor has certain other major limitations, including use of materials unsuitable for corrosive liquids. (Ryton and epoxy for example). Also, molded parts have to be resized to accommodate various flow ranges, thereby making it economically unadaptable for measuring a wide variety of flow ranges.
Various other companies manufacture paddle wheel flow-sensors capable of measuring liquid flows of very aggressive chemicals. However, the sensors are quite large, will not respond to very low liquid flow rates, and are generally totally unsuited for the measurement of gaseous flows.
Certain gas flow sensors, commonly referred to as mass flow sensors, can measure very low gas flow rates with high precision. These sensors currently use stainless steel flow-through tubes that are heated so that they are limited as to the type of gas (compatible with stainless steel) they can measure. Further, they are not adaptable to blends of gases wherein the percentage of two or more gases that are blended together are allowed to vary. This is because the detection principle is based upon the specific heat of each gas and a calibration must be made for each particular type of gas to be measured. When a blend of gases is measured, wherein the ratio of the gases varies, the flow can then obviously not be measured with any great degree of accuracy since the detection principle and specific heat of the blend gas will vary with the composition of the gas flow. Further, thermal mass flow gas sensors will not measure liquid flows. Finally, these sensors typically utilize a wheatstone floating bridge design in the detection device so that they must be "warmed-up" and zeroed prior to use. This can introduce a fairly large time delay and zero error. Thus, these mass flow sensors are limited in their usage and generally require very pure, particle-free gases to prevent malfunction.
The devices of U.S. Pat. No. 4,467,660 comes the closest to meeting all the requirements for a flow measuring transducer, except that they are unable to handle aggressive gases and liquids due to the materials of construction used. Typically, commercial devices under this patent utilize RYTON.RTM. R-4 (a 40% glass-filled polyphenylene sulfide) which is a strong engineering plastic with very low thermal coefficient of expansion. This makes the measuring device thermally stable. However, this material and others used in that design are not immune to attack by chlorine gas, hydrochloric acid, and other aggressive chemicals.
There yet exists a need for a flow sensor that can be used to measure flow rates of chemically aggressive fluids and that presents inert surfaces to the measured fluid to prevent contamination of the fluid. Additionally, the flow measuring transducer should be low in energy consumption and have no "warm-up" time delay. Further, the flow sensor should readily be adaptable to measure flow rates in different ranges (from 20 to 10,000 ml/min (liquids and gases)) with the desired degree of accuracy.